In a system enabling MIMO techniques like long-term evolution (LTE) system, MIMO techniques contribute to improve frequency efficiency and network capacity in the 3rd Generation Partnership Project (3GPP). For instance, in transmission mode (TM) 3, TM4 and TM8 of Rel-9, two layer transmissions are scheduled for a single user as single user MIMO (SU-MIMO). Alternatively, two or more user equipments (UEs) can be paired together to share the same time-frequency resources as MU-MIMO. MU-MIMO exploits further the spatial separation and diversity, and higher frequency efficiency than SU-MIMO is expected, which however may not be the case in practice.
Due to the limitation of UE's antenna and processing capability, a typical scenario of MU-MIMO is to schedule each of paired UEs with partial layers. The remained layers are disabled as dummy for other paired UEs. Taking single-layer DL MU-MIMO as example, there are two layers available for two UEs. Usually, only one layer is scheduled for each of paired UEs as shown in FIG. 1.
In conventional MU-MIMO, since the paired UEs are not well separated from each other, each UE suffers strong inter-layer interference from other paired UEs. Usually, opposing to SU-MIMO where joint detection (e.g. minimum mean square estimation (MMSE) or interference rejection combining (IRC) receiver) is used, the interference from a paired UE in MU-MIMO is simply taken as noise without doing joint detection (JD) as done in SU-MIMO, which results in throughput degradation as observed in field test.
In order to improve the MU-MIMO performance, several solutions have been proposed as below:
1) Pair UEs that have Good Spatial Separation
The problem is that spatial separation in DL is difficult to be estimated by eNodeB (eNB). And the pairing rate will be degraded in order to obtain good multiple-user separation. So the performance gain is limited by existing interference and low pairing rate.
2) Design Null Beamforming Weight
The weights for paired UEs can be designed carefully to null the interference. However, the weights are calculated based on the uplink (UL) channel estimation. Due to the channel estimation inaccuracy and non-ideal channel reciprocity, it's difficult to separate well such DL inter-layer interference by eNB.
On the other hand, when null beamforming weight is used, the power of desired signal is degraded compared with that of maximum ratio combining (MRC) and grid of beam (GOB) weights. Furthermore, the computation complexity for of nulling space processing is another challenge for eNB.
3) Do Blind IRC at UE Side
Some advanced UEs have the capability to do blind IRC to mitigate the unknown interference. However, it can not resolve the inter-layer interference well in DL MU-MIMO, among other reasons, blind IRC is not supported by all UEs in any cases. Receiver algorithm is a UE-specific behavior, which is not mandatory by 3GPP. IRC might not be supported or enabled by all UE vendors in any cases, due to the complicated processing, various scenarios and etc. Thus, it can not assume IRC working well at UE side when doing DL MU-MIMO.
4) Do Blind Detection on Presence of Interfering Layers
As specified in 3GPP and descried in the patent application No. US20100285810, the UE-specific reference signal for port7, port8 and port v+6 is independent on UE-specific radio network temporary identifier (RNTI) and length of Physical Resource Block (PRB) allocated. This allows UE to detect blindly if other layers are being co-scheduled for other UEs, and do channel estimation for MMSE or IRC accordingly. It requires UE to do blind detection on presence of interference on each PRB by searching all possible reference sequences. It introduces extra complexity for UE. Furthermore, this kind of blind detection is not robust enough, with possibility of either false alarm or missing, due to the non-perfect orthogonality of reference sequence. In addition, in common reference signal (CRS) based transmission mode (e.g. TM4), the blind detection on presence of interfering layers is infeasible.